spork
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Thanks again, but I haven't made or lost any money on it. I approached Google and Joby Energy to sponsor it, and happily they did.
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Munkul
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The reason a Helicopter needs gravity, is the pendulum effect stability it gets from the body under the rotors. Sure it will still move in zero gravity as it uses atmosphere for propulsion, but a good strong gust of wind would blow it onto its side or even flip it, without the inherent stability of its mass to dampen and reduce external forces. Not even sure it would be controllable at all after leaving the ground, as any sort of air speed would push against the body and rotate it without mass to counteract it.
It's not about trick wind flaps or clever aerillons etc, it's just about basics IMO.
The Harrier jump jet always amazed me that they even put it into service. When it had low air speed for VTOL/STOL, it had zero inherent stability, and was relying completely on the four thrusters.
A gust of wind and a momentary mistake by the pilot, and it's upside down... An amazing technical feat that has never been entirely replicated... these days I think electronics could play a much larger part in stabilizing and making it easier and safer to fly.
It's not about trick wind flaps or clever aerillons etc, it's just about basics IMO.
The Harrier jump jet always amazed me that they even put it into service. When it had low air speed for VTOL/STOL, it had zero inherent stability, and was relying completely on the four thrusters.
A gust of wind and a momentary mistake by the pilot, and it's upside down... An amazing technical feat that has never been entirely replicated... these days I think electronics could play a much larger part in stabilizing and making it easier and safer to fly.
spork
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This is a common misconception. Helicopters with a rigid or semi-rigid rotor system (anything but a teetering rotor) have no pendular stability. Like a rocket, the rotor creates what's known as a "following force". When the helicopter tips, the force tips with it. It no longer points straight up. So helicopters tend to be neutrally stable right-side-up or upside-down. I've never flown a full-scale helicopter upside-down, but I've flown radio controlled helis upside down LOTS. Whether flying around inverted, doing outside loops, or hovering inverted in front of my face, it feels exactly the same as it does flying it right-side-up... with the exception that you have to learn which controls reverse until it becomes second nature. But neither orientation is more or less stable.
Indeed. It's not a question of propulsion. It's a question of dynamics and control. If you had both a tail rotor and a "nose-rotor" equidistant from the main rotor, you could stabilize it pretty easily since those rotors would generate the same torque but opposing side-force that cancels out. But you'd still have to tip the main rotor all the way into the direction you wanted to move - rather than just tilting it a bit forward to go forward.
Yup - but lots of military jets are actually designed with negative stability and a fly-by-wire system between the pilot and controls that create a synthetically stable aircraft. This makes it easier to have an insanely responsive aircraft.
Munkul
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Pendulum was the wrong word. Take that out, and it all makes sense
Like you say, when the helicoptor's mass is in line with gravity, it is neutrally stable. Take the gravity away, and there is then no stability. Nothing to counteract sideways forces. You could get it steady state when there is zero wind, but if you wanted to fly a distance the rotors would end up being 90 degrees to direction of travel. Either which way the body was pointing.
But any gust of wind sideways would blow it off course.
If I'm wrong, teach me. Because if i'm wrong, you're then saying a helicopter also doesn't need gravity at all? (I haven't actually googled it to see what the answer is)
spork
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Helicopters are more or less neutrally stable in any attitude. But of course they'll be falling if you put them in a non-vertical attitude while not in an accelerated state (like a coordinated turn).
I don't follow.
The helicopter does need gravity, while an airplane does not. But the reason is that the tail-rotor produces an unbalanced force. With gravity you can balance the sideways force of the tail rotor by tilting the main rotor slightly to the opposite side. You can see this tilt when a helicopter is hovering.
Munkul
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This is what I'm picturing. Top - normal. Bottom - zero gravity, with zero wind (just airspeed - as the copter moves through the air, the body will take the path of least resistance)
spork
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Right. This is one of the two effects I was trying to describe. With gravity you just tilt a bit forward to go forward. Without gravity you have to tilt 90 deg forward.
The other problem isn't so easy. You have an unbalanced side force from your tail rotor that can be countered by tilting the main rotor slightly when you have gravity; but like this, you have to tilt the main rotor 90 deg to counter it without gravity. But that just tilts the tail rotor force by that same 90 deg. So you're kind of like a dog chasing its tail.
Munkul
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So this is the scenario i was trying to describe. Steady state moving in air, but any gust would blow the copter off course, the effect being large at low travel speed and probably minimal at high speed due to the air resistance stabilizing the body. The gust may come from the side or top or bottom or basically any direction except the direction of travel (or reverse)
My other point is that without gravity, the helicopter needs very little lift to move - so all that savage downdraft airflow that the blades usually generate will be much less, and the effect of wind gusts magnified.
So yes, not a pendulum effect, but the mass itself is the stabilizing element.
I hadn't thought about or understood the tail rotor issue, but I do now. I think you could counter it in acceleration through the air, but staying still in the air would be impossible... may not even work at steady-state speed either?
My other point is that without gravity, the helicopter needs very little lift to move - so all that savage downdraft airflow that the blades usually generate will be much less, and the effect of wind gusts magnified.
So yes, not a pendulum effect, but the mass itself is the stabilizing element.
I hadn't thought about or understood the tail rotor issue, but I do now. I think you could counter it in acceleration through the air, but staying still in the air would be impossible... may not even work at steady-state speed either?
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Screwdriver
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I don't know th first thing about helicopters other than that they are an abomination that really ought not to be in the air. I think @spork has already confirmed the mass of the body has a negligible effect on spaceflight (!) since the flight dynamics are (surprisingly!) "neutral". As I understand it, it is the stabilising effect from the tail rotor which in zero g does not "balance" when you tilt it because there is no gravity to tilt against.
daleyd
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Thanks, I’ve found your posts very interesting!!
